Alloy steel powder and their sintered body

ABSTRACT

A steel powder and their sintered body comprise iron as its primary component and further comprise from 1.4 to 2.0% by weight of carbon, less than 1.0% by weight of silicon, less than 1.0% by weight of manganese, from 11.0 to 13.0% by weight of chromium, from 0.3 to 2.3% by weight of titanium, less than 0.75% by weight of a combination of copper and nickel, and less than 5.0% by weight of at least one strengthening element. During sintering, titanium carbide inhibits grain coarsening, whereby the sintering window can be expanded to about 50° C.

FIELD OF THE INVENTION

The present invention relates to a steel powder and their sintered body, particularly to an alloy steel powder applied to powder metallurgy and a sintered body made of the alloy steel powder.

BACKGROUND OF THE INVENTION

Powder metallurgy has been extensively used to fabricate various metal products, especially machine parts. In the conventional powder metallurgy process, metal powders are mixed and then pressed to form a green compact; next, the green compact is sintered at high temperatures, whereby the atoms diffuse and join particles to form a sintered body with a high density. With the advances of science and technology, the demands in shape complexity and dimensional precision of the machine parts also increase. These demands can now be met by using metal injection molding (MIM) process, which combines the techniques of powder metallurgy and plastic injection molding to fabricate high-complexity workpieces with high density and superior mechanical properties.

In the MIM process, metal powders and binders are mixed to obtain a feedstock; the feedstock is injected into a mold to form a green compact with an injection molding machine; the green body is ejected and then sintered to produce a sintered body. The powders used in the MIM process are usually low alloy steel powders or stainless steel powders, which are suitable for fabricating structural parts for electronic devices that do not require high hardness. When high hardness and strength are required, the parts are usually made of tool steels. For example, the hinges of a notebook computer normally require a hardness of HRC58. Thus, tool steels, such as SKD11 (JIS, Japanese Industrial Standards) and D2 (AISI, American Iron and Steel Institute) are frequently used for this application. These tool steels have a martensitic matrix containing a great amount of carbide and thus have superior hardness and wear resistance. The standard sintering method for these tool steel powders is SLPS (Supersolidus Liquid Phase Sintering), which sinters parts at a temperature range above the solidus and below the liquidus. The adequate temperature ranges for these tool steel powders are usually between 5° C. and 10° C. When it is below the temperature range, there is not enough liquid phase and the matrix is austenitic in which atoms diffuse slowly. As a result, the sintered density is low. When the sintering temperature is above the temperature range, too much liquid phase forms and causes distortion. Furthermore, grains coarsen and brittle carbide network forms. Thus the sintered body has poor mechanical properties. These sintering behaviors demonstrate that tool steels such as SKD11 and D2 usually have a very narrow sintering window. Thus; the production yield is quite low by using standard sintering furnaces.

U.S. Pat. No. 7,211,125 proposed a steel powder with improved degree of sintering for metal injection molding and sintered body. The steel contains 0.1-1.8 wt % carbon, 0.3-1.2 wt % silicon, 0.1-0.5 wt % manganese, 11.0-18.0 wt % chromium, and 2.0-5.0 wt % niobium, with the balance being iron and impurities. The abovementioned MIM steel powder has a sintering window of about 50° C. However, niobium is rare and expensive. Thus, fabricating parts with such an alloy steel powder is economically inefficient.

SUMMARY OF THE INVENTION

The primary objective of the present invention is to overcome the problem of high material cost resulting from the fact that the conventional alloy steel powder cannot have a wider sintering window unless it contains 2.0-5.0 wt % niobium.

To achieve the above-mentioned objective, the present invention provides a steel powder and their sintered body. The steel powder comprises iron as its primary component and further comprises from 1.4 to 2.0% by weight of carbon, less than 1.0% by weight of silicon, less than 1.0% by weight of manganese, from 11.0 to 13.0% by weight of chromium, from 0.3 to 2.3% by weight of titanium, less than 0.75% by weight of a combination of copper and nickel, and less than 5.0% by weight of at least one strengthening element.

In the present invention, titanium combines with carbon to form titanium carbide during sintering, which effectively inhibits grain coarsening and therefore decreases the thickness of the liquid layer between steel powders. As a result, the sintering window expands to about 50° C. In comparison with conventional materials, the present invention only requires from 0.3 to 2.3% by weight of titanium, whereby the material cost can be reduced and the yield increased. Further, the present invention can refine the grain size to improve mechanical properties, such as strength, hardness and toughness.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the relationship of the sintering temperature and density of the sintered body in Example 1 of the present invention.

FIG. 2 is a graph showing the relationship of the sintering temperature and density of the sintered body in Example 2 of the present invention.

FIG. 3 is a graph showing the relationship of the sintering temperature and density of the sintered body in Example 3 of the present invention.

FIG. 4 is a graph showing the relationship of the sintering temperature and density of the sintered body in Example 4 of the present invention.

FIG. 5 is a graph showing the relationship of the sintering temperature and density of the sintered body in Example 5 of the present invention.

FIG. 6 is a graph showing the relationship of the sintering temperature and density of the sintered body in Example 6 of the present invention.

FIG. 7 is a graph showing the relationship of the sintering temperature and density of the sintered body in Comparative Example 1.

FIG. 8 is a graph showing the relationship of the sintering temperature and density of the sintered body in Comparative Example 2.

FIG. 9 is a graph showing the relationship of the sintering temperature and density of the sintered body in Comparative Example 3.

FIG. 10 is a graph showing the relationship of the sintering temperature and density of the sintered body in Comparative Example 4.

FIG. 11 is an electronic microscope image of the sample sintered at a temperature of 1260° C. in Example 1.

FIG. 12 is an electronic microscope image of the sample sintered at a temperature of 1240° C. in Comparative Example 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The technical contents of the present invention are described in detail in cooperation with the drawings below.

The steel powder of the present invention comprises iron (Fe) as its primary component and further comprises from 1.4 to 2.0% by weight of carbon (C), less than 1.0% by weight of silicon (Si), less than 1.0% by weight of manganese (Mn), from 11.0 to 13.0% by weight of chromium (Cr), from 0.3 to 2.3% by weight of titanium (Ti), less than 0.75% by weight of a combination of copper (Cu) and nickel (Ni), and less than 5.0% by weight of at least one strengthening element. The strengthening element may be molybdenum (Mo), vanadium (V), tungsten (W) or a combination thereof, and is preferably from 0.2 to 1.7% by weight. In the present invention, a source of titanium in the steel powder may be from titanium powder, a titanium-containing pre-alloyed powder, a titanium-containing carbide powder, or the titanium is already in the liquid metal during melting prior to powder atomization. The titanium-containing carbide powder may be a titanium carbide powder, a tungsten-doped titanium carbide powder, or a vanadium-doped titanium carbide powder. The preferred average grain size of the titanium-containing carbide powder is less than 5 μm. Besides, a source of carbon in the steel powder may be from graphite or carbon black.

In the present invention, titanium combines with carbon to form titanium carbide, which can effectively inhibit grain coarsening during sintering and prevent the sintered body from serious deformation after sintering. Therefore, the steel powder of the present invention can be sintered to high relative density in a wider temperature range and the sintered body can have higher dimensional stability. Further, the grain refinement can greatly promote the mechanical properties of the sintered body (such as strength, hardness and toughness). When the concentration of titanium is less than 0.3% by weight, the abovementioned effect is not obvious. When the concentration of titanium is higher than 2.3% by weight, the sintered body is hard to achieve desired densification.

Carbon and carbide can increase strength and hardness of the sintered body. When the concentration of carbon is less than 1.4% by weight, the sintered body cannot have desired wear resistance since the amount of chromium carbide is low. When the concentration of carbon is higher than 2.0% by weight, toughness of the sintered body decreases. Manganese can improve the hardness of the sintered body due to its high hardenability. However, manganese tends to combine with oxygen when the steel powder is manufactured by an atomization method. Too much manganese will make the steel powder contain too much oxygen, and this will cause decarburization during sintering. Therefore, the concentration of manganese is preferred to be lower than 1.0% by weight.

After sintering, chromium exists in the form of solid solution in the matrix and thus improves the corrosion resistance of the sintered body. Chromium can also combine with carbon to form chromium carbide to improve the hardness of the sintered body. The concentration of chromium in the present invention is preferably from 11.0 to 13.0% by weight. Nickel and copper exist in the form of the solid solution in the matrix, improving the strength of the sintered body via solid-solution strengthening. The concentration of nickel plus copper is preferably less than 0.75% by weight. When the steel powder is manufactured by an atomization method, silicon will combine with oxygen to form an oxide layer on the surface of particles and prevent the powder from being over-oxidized. When there is too much silicon, the oxide layer will be too thick, which impairs sintering. Therefore, the concentration of silicon is preferably less than 1.0% by weight.

The strengthening element, such as molybdenum, vanadium, tungsten or a combination thereof, combines with carbon to form carbide and improves the hardness of the sintered body. Lower than 0.2% by weight of the strengthening element has limited hardness increasing effect. When the concentration of the strengthening element is greater than 1.7% by weight, the hardening effect decreases. Therefore, the concentration of the strengthening element is preferred from 0.2 to 1.7% by weight.

The examples given below are used to further demonstrate the present invention. However, the examples are only to exemplify but not to limit the scope of the present invention. The chemical compositions of Examples 1-6 and Comparative Examples 1-4 listed in Table.1 were used to make parts by using the metal injection molding (MIM) process. The densities and deformations of these samples sintered at different temperatures were measured.

In the following Examples and Comparative Examples, the steel powder and an appropriate amount of binder were mixed in a Z-blade mixer to form granular feedstock at a temperature of 150° C. for 1 hour, wherein the weight of the binder was 7% by weight. Next, the feedstock was put into an injection molding machine to produce column-shaped samples each having a diameter of 12.5 mm and a height of 20 mm. Next, the samples were debinded in a vacuum furnace, wherein the temperature was raised to 650° C. at a rate of 5° C./minute and then maintained at 650° C. for 1 hour. Next, the temperature is raised to a specified sintering temperature at a rate of 10° C./minute and maintained at the specified temperature for 1 hour for sintering. Then, the temperature is lowered down to obtain the sintered bodies. Herein, the steel powder of the present invention is exemplified with the MIM process. In practical applications, the steel powder of the present invention may also be used for other processes, such as press-and-sinter process.

The density of the sintered bodies was measured using the Archimedean method. FIGS. 1-6 and FIGS. 7-10 show respectively the densities of the samples sintered at different temperatures in Examples 1-6 and Comparative Examples 1-4. The measured densities and the theoretical densities are listed in Table.2. The relative densities calculated using the data in Table.2 are listed in Table.3. The degree of deformation was evaluated by measuring the diameters at two ends of each sample and the obtained data are listed in Table.4. When the difference of the diameters at the two ends was greater than 1%, the sample was marked by X, which means that the dimensions of the sample are unqualified. When the difference of the diameters at the two ends was less than 1%, the sample was marked by 0, which means that the dimensions of the sample are qualified. When the relative sintered density is 98% or higher and the dimensions are qualified, the sintering temperature used, including a ±5° C. measurement error, is considered adequate.

TABLE 1 Chemical Compositions of Examples 1-6 and Comparative Examples 1-4 (wt %) C Si Mn Cr Mo V W Ti Ni Cu Fe Example 1 1.60 0.28 0.51 12.50 1.08 0.43 — 0.68 0.33 — balance Example 2 1.54 0.32 0.39 12.06 1.00 0.35 — 0.80 0.10 0.01 balance Example 3 1.68 0.32 0.39 12.06 1.00 0.35 — 1.60 0.10 0.01 balance Example 4 1.53 0.32 0.39 12.06 1.00 0.35 0.93 0.80 0.10 0.01 balance Example 5 1.64 0.29 0.36 11.71 0.98 0.61 — 1.00 0.22 0.05 balance Example 6 1.52 0.29 0.36 11.71 0.98 0.61 — 2.00 0.22 0.05 balance Comparative Example 1 1.52 0.32 0.39 12.06 1.00 0.35 — — 0.10 0.01 balance Comparative Example 2 1.48 0.29 0.36 11.71 0.98 0.61 — — 0.22 0.05 balance Comparative Example 3 1.47 0.32 0.39 12.06 1.00 0.35 1.90 — 0.10 0.01 balance Comparative Example 4 1.55 0.32 0.39 14.93 1.00 0.35 — — 0.10 0.01 balance

TABLE 2 Densities (g/cm³) of Samples Sintered at Different Temperatures in Examples 1-6 and Comparative Examples 1-4 Theo- ret- ical Den- Sintering Temperature sity 1190° C. 1200° C. 1210° C. 1220° C. 1230° C. 1240° C. 1250° C. 1260° C. 1270° C. 1280° C. 1290° C. Example 1 7.76 — — — 6.69 7.71 7.71 7.72 7.73 7.72 7.72 — Example 2 7.76 — — — — 7.14 7.68 7.70 7.68 7.68 — — Example 3 7.74 — — — — 6.24 7.68 7.68 7.70 7.69 7.70 7.65 Example 4 7.76 — — — 6.55 7.36 7.71 7.70 7.70 — — — Example 5 7.74 — — — — 6.34 7.69 7.68 7.69 7.68 7.68 7.68 Example 6 7.74 — — — — — 6.58 7.21 7.71 7.72 7.72 7.70 Com- 7.80 — — 6.41 7.11 7.57 7.72 7.72 — — — — parative Example 1 Com- 7.80 — — 6.53 7.23 7.58 7.72 7.71 — — — — parative Example 2 Com- 7.92 5.73 7.13 7.78 7.77 — — — — — — — parative Example 3 Com- 7.74 — — — 6.98 7.53 7.71 7.71 — — — — parative Example 4

TABLE 3 Relative Densities of Samples Sintered at Different Temperatures in Examples 1-6 and Comparative Examples 1-4 Sintering Temperature 1190° C. 1200° C. 1210° C. 1220° C. 1230° C. 1240° C. 1250° C. 1260° C. 1270° C. 1280° C. 1290° C. Example 1 — — — 86.2% 99.4% 99.4% 99.5% 99.6% 99.5% 99.5% — Example 2 — — — — 92.0% 99.0% 99.2% 99.0% 99.0% — — Example 3 — — — — 80.6% 99.2% 99.2% 99.5% 99.4% 99.5% 98.8% Example 4 — — — 84.4% 94.8% 99.4% 99.2% 99.2% — — — Example 5 — — — — 81.9% 99.4% 99.2% 99.4% 99.2% 99.2% 99.2% Example 6 — — — — — 85.0% 93.1% 99.6% 99.7% 99.7% 99.5% Comparative — — 82.2% 91.1% 97.0% 99.0% 99.0% — — — — Example 1 Comparative — — 83.7% 92.7% 97.1% 99.0% 98.3% — — — — Example 2 Comparative 72.3% 90.0% 98.2% 98.1% — — — — — — — Example 3 Comparative — — — 90.2% 97.3% 99.6% 99.6% — — — — Example 4

TABLE 4 Deformation Evaluation of Samples Sintered at Different Temperatures in Examples 1-6 and Comparative Examples 1-4 Sintering Temperature 1190° C. 1200° C. 1210° C. 1220° C. 1230° C. 1240° C. 1250° C. 1260° C. 1270° C. 1280° C. 1290° C. Example 1 — — — ◯ ◯ ◯ ◯ ◯ ◯ X — Example 2 — — — — ◯ ◯ ◯ ◯ X — — Example 3 — — — — ◯ ◯ ◯ ◯ ◯ ◯ X Example 4 — — — ◯ ◯ ◯ ◯ X — — — Example 5 — — — — ◯ ◯ ◯ ◯ ◯ ◯ X Example 6 — — — — — ◯ ◯ ◯ ◯ ◯ X Comparative — — ◯ ◯ ◯ ◯ X — — — — Example 1 Comparative — — ◯ ◯ ◯ ◯ X — — — — Example 2 Comparative ◯ ◯ ◯ X — — — — — — — Example 3 Comparative — — — ◯ ◯ ◯ X — — — — Example 4

Example 1

In this example, the pre-alloyed powder was prepared by an atomization method. The titanium is added during melting and thus, already present in the atomized powder. FIG. 1 shows that the sintered sample achieved a relative density of 98% or more in a temperature range between 1230° C. and 1280° C. The sintered sample also attained qualified dimensions in a temperature range between 1230° C. and 1270° C. Therefore, the sintering window thereof is 50° C. (1225-1275° C.). In this example, the sintered sample contains 1.6% by weight of carbon.

Example 2

In this example, the pre-alloyed powder of Comparative Example 1 was mixed with 1.0% by weight of titanium carbide (TiC) powder. FIG. 2 shows that the sintered sample achieved a relative density of 98% or more in a temperature range between 1240° C. and 1270° C. The sintered sample also attained qualified dimensions in a temperature range between 1240° C. and 1260° C. Therefore, the sintering window thereof is 30° C. (1235-1265° C.). In this example, the sintered sample contains 1.54% by weight of carbon.

Example 3

In this example, the pre-alloyed powder of Comparative Example 1 was mixed with 2.0% by weight of titanium carbide (TiC) powder. FIG. 3 shows that the sintered sample achieved a relative density of 98% or more in a temperature range between 1240° C. and 1290° C. The sintered sample also attained qualified dimensions in a temperature range between 1240° C. and 1280° C. Therefore, the sintering window thereof is 50° C. (1235-1285° C.). In this example, the sintered sample contains 1.68% by weight of carbon.

Example 4

In this example, the pre-alloyed powder of Comparative Example 1 was mixed with 2.0% by weight of a titanium-containing composite carbide powder containing 50% by weight of titanium carbide (TiC) powder and 50% by weight of tungsten carbide (WC) powder. FIG. 4 shows that the sintered sample achieved a relative density of 98% or more in a temperature range between 1240° C. and 1260° C. The sintered sample also attained qualified dimensions in a temperature range between 1240° C. and 1250° C. Therefore, the sintering window thereof is 20° C. (1235-1255° C.). In this example, the sintered sample contains 1.53% by weight of carbon.

Example 5

In this example, the pre-alloyed powder of Comparative Example 2 was mixed with 2.0% by weight of FeTi titanium-containing pre-alloyed powder containing 50% by weight of iron and 50% by weight of titanium. FIG. 5 shows that the sintered sample achieved a relative density of 98% or more in a temperature range between 1240° C. and 1290° C. The sintered sample also attained qualified dimensions in a temperature range between 1240° C. and 1280° C. Therefore, the sintering window thereof is 50° C. (1235-1285° C.). In this example, the sintered sample contains 1.64% by weight of carbon.

Example 6

In this example, the pre-alloyed powder of Comparative Example 1 was mixed with 2.0% by weight of titanium powder. FIG. 6 shows that the sintered sample achieved a relative density of 98% or more in a temperature range between 1260° C. and 1290° C. The sintered sample also attained qualified dimensions in a temperature range between 1260° C. and 1280° C. Therefore, the sintering window thereof is 30° C. (1255-1285° C.). In this example, the sintered sample contains 1.52% by weight of carbon.

Comparative Example 1

In this comparative example, the pre-alloyed powder was prepared by an atomization method and had a composition similar to that of the commercial tool steel SKD11. FIG. 7 shows that the sintered sample achieved a relative density of 98% or more in a temperature range between 1240° C. and 1250° C. The sintered sample also attained qualified dimensions at a temperature of about 1240° C. Therefore, the sintering window thereof is 10° C. (1235-1245° C.). In this comparative example, the sintered sample contains 1.52% by weight of carbon.

Comparative Example 2

In this comparative example, the pre-alloyed powder was prepared by an atomization method and had a composition similar to that of the commercial tool steel D2. FIG. 8 shows that the sintered sample achieved a relative density of 98% or more in a temperature range between 1240° C. and 1250° C. The sintered sample also attained qualified dimensions at a temperature of about 1240° C. Therefore, the sintering window thereof is 10° C. (1235-1245° C.). In this comparative example, the sintered sample contains 1.48% by weight of carbon.

Comparative Example 3

In this comparative example, elemental powders were used, and tungsten was sourced from 2% by weight of tungsten carbide (WC) powder. FIG. 9 shows that the sintered sample achieved a relative density of 98% or more in a temperature range between 1210° C. and 1220° C. The sintered sample attained qualified dimensions at a temperature of about 1210° C. Therefore, the sintering window thereof is 10° C. (1205-1215° C.). In this comparative example, the sintered sample contains 1.47% by weight of carbon.

Comparative Example 4

In this comparative example, elemental powders were used, and chromium was sourced from 2% by weight of chromium carbide (Cr₃C₂) powder. FIG. 10 shows that the sintered sample achieved a relative density of 98% or more in a temperature range between 1240° C. and 1250° C. The sintered sample attained qualified dimensions at a temperature of about 1240° C. Therefore, the sintering window thereof is 10° C. (1235-1245° C.). In this comparative example, the sintered sample contains 1.55% by weight of carbon.

In Comparative Examples 1-4, the sintering window is only about 10° C. However, the sintering window of the steel powder presented in this invention increased to a range between 20° C. and 50° C. in Examples 1-6. FIG. 11 shows an electronic microscope image of the sample Example 1 sintered at 1260° C. It is seen that the average grain size is about 11 μm. In comparison, FIG. 12 shows an electronic microscope image of the sample sintered at 1240° C. in Comparative Example 1. From FIG. 12, it is seen that the average grain size is about 86 μm. Therefore, the addition of titanium in the present invention can effectively inhibit the grain growth.

In the present invention, titanium combines with carbon to form titanium carbide and inhibits grain coarsening during sintering. Thereby, the steel powder of the present invention can be sintered into a high-density body having superior dimensional stability in a wider sintering window of 50° C. The present invention only requires from 0.3 to 2.3% by weight of titanium. Besides, titanium is easier to obtain. Therefore, the present invention has lower material cost. Further, the present invention can refine the grains of the sintered body and promote the mechanical properties thereof.

The embodiments described above are only to exemplify the present invention but not to limit the scope of the present invention. Any equivalent modification or variation according to the spirit of the present invention is to be also included within the scope of the present invention. 

1. A steel powder, comprising iron as its primary component and further comprising from 1.4 to 2.0% by weight of carbon, less than 1.0% by weight of silicon, less than 1.0% by weight of manganese, from 11.0 to 13.0% by weight of chromium, from 0.3 to 2.3% by weight of titanium, less than 0.75% by weight of a combination of copper and nickel, and less than 5.0% by weight of at least one strengthening element.
 2. The steel powder according to claim 1, wherein the strengthening element is selected from a group consisting of molybdenum, vanadium, tungsten, and a combination thereof.
 3. The steel powder according to claim 1, wherein the strengthening element is from 0.2 to 1.7% by weight.
 4. The steel powder according to claim 1, wherein a source of carbon is from graphite or carbon black.
 5. The steel powder according to claim 1, wherein a source of titanium is from titanium powder.
 6. The steel powder according to claim 1, wherein a source of titanium is from a titanium-containing carbide powder.
 7. The steel powder according to claim 1, wherein a source of titanium is from a titanium-containing pre-alloyed powder.
 8. The steel powder according to claim 6, wherein the titanium-containing carbide powder is selected from a group consisting of a titanium carbide powder, a tungsten-doped titanium carbide powder, and a vanadium-doped titanium carbide powder.
 9. The steel powder according to claim 6, wherein the titanium-containing carbide powder has an average grain size of less than 5 μm.
 10. A sintered body made of the steel powder according to claim
 1. 